The present invention relates to an audio teleconferencing apparatus which allows an audio conference to be held through the use of two-wire circuits such as telephone lines.
An audio teleconferencing apparatus has advantages, as a means for teleconferencing among a plurality of distantly located parties. Such advantages include instantaneous and convenient operations without the necessity of any traveling expenses so that several parties may confer among themselves. It offers, for example, a so-called "chattering square" or "party line" through which an unspecified number of speakers can enjoy easy talking among themselves. Hence, there is currently a strong and growing demand for the audio teleconferencing services provided by such apparatuses.
The audio teleconferencing, apparatus usually employs a public telephone network (a two-wire circuit) and is always connected to the telephone line, and teleconferencing can be started by making a telephone call to the audio teleconferencing apparatus. During teleconferencing speaker's voices pass through the telephone line twice and are reduced in volume accordingly; hence, the audio teleconferencing apparatus needs to be equipped with an amplifying function for compensating for the reduced speech volume and amplifies and adds the voices through a two-wire/four-wire conversion. At this time, a plurality of four-wire multiplex loops are formed through transformers for the two-wire/four-wire conversion in the audio teleconferencing apparatus and it is necessary that neither oscillation nor singing be allowed to occur in any of the four-wire multiplex loops. That is, the amount of amplification depends on how much the loop gains of the four-wire multiplex loops are reduced. As a solution to this problem, there have been proposed a method using an echo canceller and a method called a level priority method.
The method using an echo canceller reduces the loop gains of the four-wire multiplex loops by cancelling sneak paths in the two-wire/four-wire conversion circuit of the audio teleconferencing apparatus by the use of the echo canceller. With this method, however, a sufficient amount of amplification cannot be obtained.
According to the level priority method, the electric power of voices of a plurality of speakers participating in the audio teleconference is compared and only the speaker's voice, of the maximum electric power is allowed to be heard by all participants but the other speakers' voices are blocked. This is a system that selectively switches speakers' voices according to their loudness. Since all four-wire loops are thus cut off, desired amplification can be achieved, but since switching control is effected on the basis of the electric power of voice, a particular speaker's voice is very likely to be switched ON according to the difference in loudness or circuit loss among speakers' voices; furthermore, this is a complete one-way system. Thus, the level priority method has a defect that communications are readily disrupted.
For example, in the case of the "chatting square," a plurality of conference rooms are prepared for individual topics and users send PB (DTMF: Dual Tone Multi-Frequency) signals during communication as they go into and out of the rooms; hence, the audio teleconferencing apparatus needs to receive the PB signals to implement the teleconferencing service. Hence, the audio teleconferencing apparatus calls for means which receives the PB signals during communication.
Conventionally, a PB receiver is provided for each channel to receive the PB signal during communication. With this method, however, the PB signal from another speaker sneaks in the two-wire/four-wire conversion circuit and is erroneously detected.
With the spread of the audio teleconferencing service intended for an unspecified number of people, such as the "chatting square," there have arisen problems of intentional interference with conversations among participants in the audio teleconference. This is very serious because intentional interference even by one of the participants would hinder conversations among the other participants and hence readily make it impossible to hold the audio teleconference. It is customary in the prior art to detect a disturbance or jamming sound on the basis of the loudness (or electric power) of the received signal; that is, the received signal is detected as a jamming sound when its electric power is large over a certain period of time. Yet, human conversations are intoned, and for instance, a shout or yell is large in sound volume but is not always a disturbance, whereas jamming sound is not necessarily large in sound volume; therefore, the conventional interference detecting method based on sound volume is erroneous and is not so high in the detection capability. Incidentally, intentional interference with the audio teleconference is usually made by continuously supplying a holding tone, music, noise or annoying sound into a telephone set connected to the audio teleconferencing apparatus.